SOLAR EV CHARGING · CHECKED 22 OCTOBER 2026
Solar charging has a cost. Make it visible.
Our 250 km week in the 2025 Tesla Model Y RWD needs 38.25 kWh at its certified 153 Wh/km baseline. If all of that came from surplus solar that would otherwise earn 5c/kWh, the forgone export income would be $1.91. The 5c rate is an editable scenario, not a retailer quote.
What does the solar electricity replace?
The useful comparison is what would happen to that energy otherwise. If it would be exported, charging gives up the feed-in payment. If it would run your home and avoid buying electricity, diverting it to the car can leave the home buying more from the grid. That trade-off uses the import rate. A home battery adds another choice with its own efficiency, capacity and ownership costs.
Solar installation is not included in this energy comparison. Calling surplus generation “free fuel” can hide both the export income and the cost of the equipment. The calculator’s home plus solar mode values solar at the feed-in tariff you enter. It does not calculate the investment return on panels or a home battery.
How much of a week can surplus solar cover?
A system’s nameplate kW is not its daily kWh output. There is no defensible single “typical system covers a week” figure without location, season, shade, orientation, household use and when the car is parked at home. Use your inverter and export records to estimate the surplus during the actual charging window.
| Surplus per available day | Five-day energy | Share of this 38.25 kWh week |
|---|---|---|
| 2 kWh | 10 kWh | 26% |
| 5 kWh | 25 kWh | 65% |
| 8 kWh | 40 kWh | 100% |
These examples assume energy can be used when generated and exclude an additional allowance. More output on a day when the car is away does not necessarily help that week’s charging. A single high-output day also cannot be averaged into several days of storage unless you actually have suitable storage.
Compare daytime solar with overnight grid energy
Suppose half of the weekly charging uses surplus solar at 5c/kWh forgone and half uses the Ausgrid reference at 33.1c/kWh. The energy cost is $7.29, compared with $12.67 for all-grid charging at that reference. Those calculations retain the tariff’s full source precision.
A special overnight or daytime plan can change that choice. Check the charging window, controlled charging rules, meter and vehicle eligibility, and how the plan prices the rest of the home. A cheap EV window can come with a higher supply charge, other usage rates or subscription. Our plan comparison keeps these restrictions beside the published rate.
Choose charging controls for your situation
Ask the installer whether the proposed charger can respond to measured surplus, what minimum charging power it needs, whether it imports grid electricity when clouds pass and which features need a paid subscription. Confirm compatibility with the car and existing solar system. A timer schedules hours; it does not necessarily measure spare generation.
The Australian Government home charging guide identifies rooftop solar and off-peak tariffs as useful home charging options. A dedicated charger needs a licensed electrician and may require electrical upgrades. Shared buildings can also require approval. Keep equipment and installation quotes separate from the energy estimate.
Put your own numbers in
- Read the feed-in tariff on your current offer and the import price in the hours you would otherwise use.
- Check how much surplus is available while the car is home, using actual generation and household records.
- Estimate the fraction of your charging energy that can use that surplus, rather than the fraction of days with sunshine.
- Use home plus solar or mixed charging in the calculator, then test a lower-surplus week as well.
The car’s certified energy figure is a baseline. For an observed week, use measured consumption and choose any extra-energy allowance with the measurement boundaries in mind. Avoid adding losses twice. Read the home charger guide before choosing installation hardware.